Proposals have recently been made to feed motor vehicle discharge lamps with squarewave alternating currents at a frequency of about 200 Hz to 1 kHz.
FIG. 1 shows a circuit for providing that type of power supply.
The circuit comprises a DC/DC converter 1 powered with DC from the vehicle battery B and a DC/AC converter 2 connected between the converter 1 and the headlight discharge lamp 3. A high voltage pulse generator module 4 is connected in series therewith for triggering the lamp 3.
The DC/AC converter 2 comprises four fast switches Q.sub.1 to Q.sub.4 connected as an H-bridge and controlled by a control circuit 6. The switches Q.sub.1 to Q.sub.4 are MOS type transistors, for example, and the control circuit 6 controls the grid voltages thereof. They must be capable of withstanding isolated voltages in the range 0 V to 500 V in the discharge lamp, and also currents in the range 0 A to 3 A, including transients that may be as great at 10 A, lasting for a few hundredths of microseconds.
These constraints require the various components of the circuit 6 controlling the switches Q.sub.1 to Q.sub.4 to be large in size. This applies in particular to transformers included in such control circuits, which transformers are dimensioned so as to be capable of withstanding voltages of 500 V.
Unfortunately, it is presently desired that power supply control circuits for discharge lamps should be considerably reduced in bulk so that such circuits can be housed completely within headlights, whereas in the past the converters of power supply circuits have been external to headlights.